1. Field of the Invention
The present invention relates to a device for holding a workpiece or a bar of a material to be machined on a rotating spindle of a machine tool, especially an automatic lathe or a profiling lathe provided with a spindle equipped with such a holding device.
2. Description of the Related Art
As modern high-productivity machine tools operate at high spindle speed and with tools having a large turning-removal capacity, the problem of keeping the workpiece or the bar in the spindle, or of holding the chuck mounted in the spindle, respectively, becomes most important.
The known purely mechanical holding devices, generally actuated by a bell-shaped cam, can no longer be used on machines such as mentioned above, seeing that the holding force must be transmitted by mechanical means, for example ball bearings.
The known purely pneumatic holding devices do not offer a sufficient holding capacity unless they are very voluminous. On the other hand, disappearance of the pressure generally causes a loosening of the chuck, which may be dangerous, as much for the machine as for the user.
To avoid the loosening of the chuck in case of disappearance of the pressure, it is known to effect the holding by means of the force of the spring means and, for the loosening, to use pneumatic means. In order to attain a sufficient holding capacity, it is necessary to use very strong spring means, which necessitates corresponding pneumatic means for the loosening.
Patent EP 0 213 075 describes such a holding device with a spring for attaining the holding force and pneumatic means for attaining the loosening force. For attaining a sufficient loosening force, the device is equipped with a hydraulic force amplifier. Seeing that the diameter of the primary pneumatic piston is relatively wide, the moment of inertia of the device is rather great, which is undesirable in a machine where the spindle is often stopped and accelerated. Moreover, one of the drawbacks of this device is that bubbles of air in the hydraulic liquid may make it impossible to remove the workpiece or the bar to be machined.